Method for verifying a surveying instrument&#39;s external orientation

ABSTRACT

Verifying surveying instrument&#39;s external orientation during a measurement process, comprising directing the imaging means onto a reference object and detecting a first photographing direction of the imaging means, taking a first image of the reference object in the first photographing direction, memorizing the first image and the first photographing direction as being indicative of the surveying instrument&#39;s external orientation, re-directing the imaging means onto the reference object and detecting a second photographing direction of the imaging means, taking a second image of the reference object in the second photographing direction, and comparing a first with a second imaged position of the reference object in the first respectively the second image by image processing as well as the first with the second photographing direction and verifying the surveying instrument&#39;s external orientation based on disparities between the first and the second imaged position and/or between the first and the second photographing direction.

The invention pertains to a surveying method and especially to a methodfor verifying a surveying instrument's external orientation according tothe preamble of claim 1 as well as to a surveying instrument accordingto the preamble of claim 14, being designed for executing the inventivemethod.

When setting up a surveying instrument such as a total station, it isnecessary to determine the surveying instrument's orientation.Generally, the orientation of a surveying instrument corresponds to thedeviation of the surveying instrument's alignment (direction zero of theangle measurement means) from the north direction (heading), thatis—according to the common geodetic convention—from an x-axis of ahorizontal coordinate system. To determine the orientation of thesurveying instrument, an angle from the surveying instrument to at leastone comparatively far distanced reference object having known coordinatedata is measured. In general, the distance of such a reference objecthas to be at least 1 km.

Examples for such reference objects are a church tower cross, a cross onthe summit of a mountain (summit cross), a high chimney or otherdistinctive objects being visibly over a long distance. Coordinate dataof these reference objects in general are provided by public authoritiese.g. the “Bundesamt für Landestopographie swisstopo” in Switzerland.

A prior art method for determining a surveying instrument's orientationis described on the basis of FIGS. 1 and 2. In order to determine thesurveying instrument's orientation, an angle O between the surveyinginstrument's direction zero and the x-axis of the coordinate system(usually corresponding to the north direction) is determined. That is,the orientation of a surveying instrument in general is defined as theangle between the x-axis (north direction) and the surveyinginstrument's direction zero.

If the coordinate data of the setup point of the surveying instrument Aand of the reference object B are known, from the coordinate data anoriented direction angle r₀ may be calculated. The orientation directionangle is the angle between the x-axis and the line S connecting theposition of the surveying instrument and the reference object B. Aftersetting up the surveying instrument, the direction angle r to thereference object, that is, the angle between the direction zero of thesurveying instrument and the line connecting the position of thesurveying instrument and the reference object B is measured and theorientation angle O is determined as:

O=r ₀ −r

In use of the surveying instrument in the field, it may happen that thesurveying instrument's external orientation changes slightly for severalreasons, e.g. because one foot of the tripod sinks into plaint ground,because of an inadvertent push or hit onto the surveying instrument orsimilar. Therefore, the surveying instrument's external orientation maybe verified during a measurement process a while after starting themeasurement tasks, i.e. in a proceeded or later state of the measurementprocess. Such a verification of the orientation may ensure that thesurveying instrument didn't change and still is exactly in the initiallydetermined orientation or, in case of a change in the orientation, mayindicate that its external orientation has to be newly determined.

For verifying the surveying instrument's orientation, after a while, thedirection angle r₂ from the surveying instrument to the reference objectB is again determined, and then the new reference angle is compared withthe originally determined direction angle r₁.

$\begin{matrix}{{\Delta \; O} = {O_{2} - O_{1}}} \\{= {\left( {r_{0} - r_{2}} \right) - \left( {r_{0} - r_{1}} \right)}} \\{= {r_{1} - r_{2}}}\end{matrix}$

If the change of the orientation ΔO exceeds a predetermined value, thesurveying instrument's orientation is to be newly determined. Therein,it may be remarked that the change of the orientation ΔO can bedetermined without knowing the absolute external orientation values O₂and O₁. However, according to methods of the state of the art, such averification method has to be carried out completely manually and is,therefore, cumbersome. Also, the reliability depends on the skill of theoperating person.

Exemplarily, WO 2009/106144 A1 roughly describes a verifying procedurefor checking if the originally determined orientation of a surveyinginstrument did or did not change during a measurement session in thefield, see page 25, lines 25-33 of WO 2009/106144 A1:

“The orientation of the instrument with respect to a known direction maybe needed when measuring position data in an absolute coordinate system.In this procedure, the reference target is used to obtain the angleorientation of the total station at the beginning of a measurementsession (as for instance obtained after a calibration procedure) andchecked before finishing the measurement session. Checking the positionof the reference target relative to the angle orientation of the totalstation ensure that the total station has not moved during themeasurement session.”

However, also the verification method described in the above-citedpassage of WO 2009/106144 A1 has to be carried out completely manuallyand is, therefore, cumbersome. An again, the reliability of such averification depends on the skill of the operating person.

More information regarding a verification of the surveying instrument'sorientation during a user's measurement session in the field is notgiven in WO 2009/106144 A1 at all, as actually the main technicalteachings of WO 2009/106144 refer to completely different subjectmatters like to a method for calibrating a surveying instrument(see—among many other passages—page 12, lines 7-12 of WO 2009/106144 A1)or—as a minor matter—to a method for searching for a reference target(see e.g. page 23, lines 20-27 of WO 2009/106144 A1), which both formcompletely different topics compared to the matter of a method forverifying a surveying instrument's external orientation during ameasurement session in the field.

Therefore, there is need for an improved method to determine and verifya surveying instrument's external orientation during a measurementsession, particularly wherein the method can be carried out with lesseffort and with high reliability—independently from the skill of theoperator.

The method of the present invention for verifying a surveyinginstrument's external orientation during a measurement process isapplicable for a surveying instrument having at least imaging means andangle measurement means for detecting an internal photographingdirection of the imaging means relative to a base of the surveyinginstrument.

Therein, the method comprises the initialization steps of

-   -   directing the imaging means onto a reference object and        detecting a first photographing direction of the imaging means,    -   taking a first image of the reference object in the first        photographing direction, and    -   memorizing the first image and the first photographing direction        as being indicative of the surveying instrument's external        orientation (exemplarily, the detected first photographing        direction may be used as peg on which to link the surveying        instrument's external orientation for the measurements to        follow).

According to the invention, in a proceeded state of the measurementprocess (e.g. after a certain time period and after a part of themeasurement tasks have been carried out or e.g. after an operator hasinadvertent hit the surveying instrument) the further, verifying stepsare performed for verification of the surveying instrument's externalorientation (i.e. for verifying if the surveying instrument still isoriented in the same manner regarding its setting):

-   -   re-directing the imaging means onto the reference object and        detecting a second photographing direction of the imaging means,    -   taking a second image of the reference object in the second        photographing direction, and    -   comparing a first with a second imaged position of the reference        object in the first respectively the second image by image        processing as well as the first with the second photographing        direction and verifying the surveying instrument's external        orientation based on disparities between the first and the        second imaged position and/or between the first and the second        photographing direction.

Therein, particularly, the verification may be performed in thefollowing manners/embodiments:

According to a first embodiment of the method, the step of re-directingmay be performed in such a way, that the second photographing directionexactly concurs with the first photographing direction. This means,after initiating the verification process, firstly the imaging means(particularly the aiming means/telescope, wherein the imaging means areintegrated) are driven to the internal angle of the first photographingdirection (i.e. the imaging means are re-directed exactly into the firstphotographing direction). After having taken the second image of thereference object in the second photographing direction, then, thesurveying instrument's external orientation may be verified just basedon a disparity between the first and the second imaged position.

According to an alternative second embodiment, the step of re-directingmay be performed in such a way, that the second photographing directionis different from the first photographing direction. The surveyinginstrument's external orientation may then be verified based on thepositions-disparity between the first and the second imaged position aswell as the direction-disparity between the first and the secondphotographing direction. Therein, the position-disparity and thedirections-disparity have to be brought into relation to each other.

Hence, according to that alternative second embodiment of the inventivemethod, it is not necessary to again exactly approach the firstphotographing direction after starting with the initialization steps,but the imaging means may be re-directed in such a way, that the secondphotographing direction roughly coincides with the first photographingdirection. This may be an advantage compared to the first embodiment,because often it may be a problem for the motorization of surveyinginstruments to really exactly approach a previously measured angle ofthe telescope a second time with sufficient accuracy (i.e. the motorizedchange of alignment of the telescope can usually be done only lessaccurate than the angle read-out).

However, it may also merely be sufficient to re-direct the imaging meansin such a way, that the reference object is still just within its fieldof view (coverage) when taking the second image into the second imagingdirection. By taking into account the difference between the first andthe second imaged positions in the first respectively the second imageand the difference between the first and the second photographingdirections, it can be verified if the external orientation has changedor not. Therefore, the difference between the first and the secondimaged positions is put into relation with the difference between thefirst and the second photographing directions. E.g., adirecting-angle-difference can be derived directly from thepositions-disparity, and that directing-angle-difference can directly becompared with the disparity between the first and the secondphotographing direction. In case that the directing-angle-differencederived from the positions-disparity deviates from the disparity betweenthe first and the second photographing direction, it is indicated thatthe surveying instrument's external orientation has changed.

According to a further, third embodiment, the step of re-directing maybe performed in such a way, that the second imaged position of thereference object in the second image (which is taken in the secondphotographing direction) exactly concurs with the first imaged positionof the reference object in the first image. Then, the surveyinginstrument's external orientation may be verified—particularlyonly—based on a disparity between the first and the second photographingdirection.

Exemplarily for that third embodiment, the step of re-directing may beperformed iteratively by successively changing the photographingdirection and successively taking and processing images until the imagedposition of the reference object exactly concurs with the first imagedposition.

Of course, the inventive method may also be carried out according tofurther, not explicitly described embodiments lying still with the scopeof the invention (e.g. combinations of the above-described embodiments).

Hence, according to the invention, verification methods for reviewing,monitoring and checking an initially set-up and given surveyinginstrument's orientation are provided, which can be carried out in asemi-automated (e.g. with guidance for an operator) or evenfull-automatic manner.

The surveying instrument may particularly be built as theodolite ortotal station, especially with motorization, and further compriseoperating means as human-machine-interface and a controller such as adata processing unit having stored program code so as to be capable toexecute the initialization and verifying steps of the above-describedinventive methods.

Particularly, the imaging means may be integrated into an aimingtelescope having a zoom lens with e.g. 30-way zoom. Therein, the lens ofthe aiming telescope may define an aiming axis, wherein thephotographing direction may be coaxial or at least substantially coaxialwith the aiming axis. The imaging means may e.g. be an on-axis camera,also being placed—in the optical path of the telescope—after the lens.Particularly, the imaging means should be calibrated with respect to theoptics of the aiming telescope and the aiming axis.

Dependent on if the surveying instrument is provided with motorizationmeans for changing the alignment and viewing direction of the imagingmeans, which may e.g. be integrated into an aiming-telescope, the stepof re-directing the imaging means exactly into the first photographingdirection may be carried out full-automatically or may be carried out byguiding an operator so as to help re-directing the imaging means e.g.exactly into the first photographing direction (see the first describedembodiment of the inventive method).

Regarding the alternative second embodiment of the inventive method, theverifying step of re-directing the imaging means onto the referenceobject and detecting a second photographing direction of the imagingmeans may also be carried out full-automatically or may be carried outby guiding an operator so as to help re-directing the imaging means ontothe reference object and await a users command for triggering thedetection of the second photographing direction of the imaging means.The third particular embodiment of the inventive method may preferablybe performed full-automatically and—as described—in iterative manner.

In general, the angle measurement means of the surveying instrument maybe designed for detecting a horizontal angle and particularlyadditionally a vertical angle as internal photographing direction of theimaging means. If so, the step of comparing the first with the secondimaged position may be performed in more detail by determining ahorizontal pixel offset—and particularly additionally a vertical pixeloffset—between the first and the second imaged positions in the firstand second image, respectively, and the surveying instrument'shorizontal external orientation—and particularly additionally itsinclination (vertical alignment)—may be verified based on the horizontalpixel offset—and particularly also the vertical pixel offset.Furthermore, dependent on the manner of carrying out the inventivemethod, also an amount of disorientation may be calculated anddetermined, the amount of disorientation particularly being horizontaland/or vertical displacement angles. E.g., regarding the firstembodiment of the verifying method, the amount of disorientation (e.g. ahorizontal offset angle about which the surveying instruments externalorientation had changed during measurements, i.e. between theinitialisation process and the verification process) can be deriveddirectly from the pixel offset between the first and the second imagedposition in the first respectively the second image (as described lateron in more detail, see FIG. 6). Regarding the second embodiment, thepixel offset has to be put into relation with the offset between thefirst and the second photographing direction. E.g., the pixel offset mayfirstly be transformed into an angle offset. Then, the amount of changein orientation may result directly as difference between and that angleoffset and the offset between the first and the second photographingdirection. At least regarding the first and second embodiment of theinventive method, the camera should be well calibrated (i.e. the cameraconstant should be known) for directly transforming/converting a pixeloffset between the imaged positions in the first respectively the secondimage into an angle offset.

Regarding the third embodiment, the imaging means are re-directed—inconnection with the verification—so that the second imaged position (inthe verification image) exactly equals the first imaged position (in theinitialisation image). Therefore, the amount of a change in orientation(which occurred timely between the initialisation process and theverification process) may result directly as an angle difference betweenthe first and the second photographing direction.

In general, the resulting amount of disorientation may be used forcorrecting the value of the current surveying instrument's externalorientation. As an example, a determined horizontal displacementangle—as amount of disorientation—can be used for correcting the currentorientation of the surveying instrument by adding said horizontaldisplacement angle to the primarily set orientation. In this case, thechanged orientation may automatically be considered for the oncomingsurveying results, and there is no necessity that the user takes othermeasures to clear the changed orientation of the surveying instrument.

Optionally, also a warning signal can be issued if the amount ofdisorientation extends beyond a preset value. In this case, the userachieves information that the orientation has been changed beyond anadmissible range. Thus, depending on the situation, the user can performa new orientation for the surveying instrument, or he can consider othermeasures to correct the false orientation. The signal issued may be anoptical signal, an audible signal, a vibrating signal or any othersignal suitable to attract the user's interest.

Referring back to the step of comparing the first with the second imagedposition, exemplarily in more detail, said step may be carried out by anedge extraction method. Here, especially a vertical edge, such as thevertical bar of a summit cross or church tower cross is accuratelydetermined in the two images, respectively, and then the distancebetween the edges in the two images is used as the basis for thecalculation of the displacement.

Alternatively, said step of comparing the first with the second imagedposition of the reference object in the first respectively the secondimage can also be carried out by template matching. Therefore, apredetermined area around the reference object in the first image cane.g. be defined as a template. The template then can be matched in thesecond image so as to correspond with the reference object within thesecond image, and a displacement of the template between its position inthe first and the second image can be determined, the displacement beingindicative of the amount of disorientation of the surveying instrument.

In general, the surveying instrument comprises a telescope defining anaiming-axis and having integrated the imaging means, wherein crosshairsare superimposed in the image indicating the aiming-axis. In that case,as a detailed example for the template matching procedure, thepredetermined area can be defined around the crosshairs in the firstimage as template and the centre of the crosshairs—which represents theaiming-axis in the first image—can be set as a reference point of thetemplate. Then, the template can be matched within the second image, anda pixel offset between

-   -   the position of the centre of the crosshairs in the second image        and    -   the position of the reference point of the matched template in        the second image        can be determined indicative of the disorientation of the        surveying instrument.

Therein, in general, the crosshairs do not necessarily have to bedisplayed in the image (i.e. overlaid/superimposed onto the image), theposition where the aiming axis would be depicted in the image may anywaybe known. In that case, e.g., the aiming may be done by means of anocular with a reticule, and the verification of the orientation(according to the invention) may be done separately with the camera, byusing the known position where the aimed point is depicted. Hence, apixel offset between the position where the aimed point is depicted andthe position where the reference object is imaged in the second imagemay be determined and an angle-offset may derived therefrom.

According to a further aspect of the invention, instead of taking andstoring only one image in connection with the initialization andverification procedures, also a series of images can be gathered (i.e.several images being captured consecutively in short time), the synopsisof the images of each series taking place of the first respectively thesecond image.

With other words, the steps of taking the first and the second image maycomprise consecutively capturing several images in the firstphotographing direction as a first series of images of the referenceobject respectively consecutively capturing several images in the secondphotographing direction as a second series of images in the in a shortpredetermined time period (e.g. around 5-20 images immediatelyconsecutively) and averaging information derivable from the images ofthe first series respectively the second series to take place ofinformation derivable from the first respectively the second image.

Particularly, the positions of the reference object in each individualimage of the first series are averaged and the positions of thereference object in each individual image of the second series of imagesare averaged and the averaged positions are taken as the firstrespectively the second imaged position.

Hence, for example, particularly in case of air flickering, theorienting-direction derived from an average of the imaged positions ofthe reference object in each of the consecutively captured images of theseries may provide for a more precise indication of the “real”orienting-direction to the reference object compared to the case ofprocessing only one single captured image.

Therefore, applying the method according to the invention in the abovemanner enables to clear out poor visibility conditions such as haze orair flickering occurring on hot days.

Particularly in connection with the initialization steps, the surveyinginstrument's orientation and optionally also its position can bedetermined in an absolute coordinate system by referencing the surveyinginstrument via several known reference objects, the positions of thereference objects being pre-known in the absolute coordinate system andthe relative directions and/or distances to the reference objects beingmeasured. Alternatively, the position of the surveying instrument mayalso be taken from a map, e.g. if set-up upon a known georeferencedpoint mark.

Particularly, if each of the above-described steps of the inventivemethods is repeated with at least two more reference objects, inaddition to the orientation, the position of the surveying instrumentcan be verified by an intersection step or by a resection step after thecertain time period. Thus, apart from the orientation, the position ofthe surveying instrument can be verified analogously, and a change ofthe surveying instrument's position can be determined or noticed if thisposition remained unchanged.

According to a further aspect of the invention, a reminder signal can beissued after a predetermined time period has passed since the initialdetermination of the surveying instrument's external orientation orsince the last orientation verification has been carried out, and theverifying steps can be carried out upon a manual command from a user orautomatically after a further predetermined time period has passed.

Also, the verifying steps can be repeated during the measurement processin predetermined time intervals, particularly wherein the verifyingsteps are automatically carried out in a repeated manner in thepredetermined time intervals.

In that case, in connection with each completion of the verifying steps,an amount of disorientation can be determined and anorientation-function of time may be derived from the amounts ofdisorientation, particularly by a regression analysis, especially bylinear regression, wherein the surveying instrument's externalorientation is plotted over time. Hence, the measurement resultsgathered in connection with the measurement process can be correctedaccording to that orientation-function, i.e. dependent on its respectivegathering point of times and said derived orientation-function.

The method of the invention advantageously enables a user to realize ifthe orientation of a surveying instrument has changed while working withthe surveying instrument. It is possible to perform the verificationautomatically, that is, after a predetermined period has passed, oralternatively, it can be performed upon a manual command. The latter canbe given in a case where the user is aware that the orientation mighthave been changed, e.g. if the surveying instrument has been subjectedto an impact.

Especially, the method of the invention enables to verify theorientation of a surveying instrument, without the need to previouslydetermine the surveying instrument's position. This latter case may beperformed, if a local coordinate system is used which is not embedded ina global coordinate system. Here, any significant point may be used asthe reference object. Hence, it has to be noted that an orientationvalue does not necessarily have to be initially determined for carryingout and making use of the verification method according to theinvention. Rather, the basic point of the invention is to provide for amethod, with which a change of orientation in the setup of the surveyingstation—which potentially may occur during measurements—can at least beregistered (and particularly also determined in its displacement value).

Summed up, according to the invention, a horizontal displacement anglecan be calculated on the basis of a horizontal image coordinate datadifference of a predetermined reference point in the two images.Therein, according to the first embodiment, it is not necessary inconnection with the verifying steps to again measure the horizontalangle from the surveying instrument to the reference object, but merelysufficient to exactly approach the stored, first aiming direction and totake the second image.

Furthermore, it is possible to additionally calculate a verticaldisplacement angle on the basis of a vertical image coordinate datadifference of the position of the predetermined reference point in thetwo images. From calculating the vertical displacement angle, apart froma horizontal and/or vertical displacement, a change of the surveyinginstrument's inclination can be determined.

As explained above, for verifying the horizontal and/or the verticaldisplacement, a defined area around crosshairs in the first image, forinstance an image section of the first image, can be defined as atemplate. Then, a reference point can be defined in the template, and inthe second image a horizontal and/or vertical displacement can bedetermined as the distance from the reference point which was defined inthe first image to the crosshairs position in the second image. In thiscase, image coordinate data of the reference point can be determined inthe second image, and the horizontal and/or vertical displacement can bedetermined from the image coordinate data.

As also already mentioned above, the method can automatically be carriedout in a repeated manner in predetermined intervals. This ensures afrequent verification of the surveying instrument's orientation andreduces a risk that the user forgets to manually verify the orientation.Alternatively, or additionally, a manual activation of the verificationis any time possible by the user.

Furthermore, a reminder signal can be issued after a predetermined timeperiod has passed since the initial determination of the orientation orsince the last orientation verification. Then, the method for verifyingthe surveying instrument's orientation can be carried out upon a manualcommand from a user, or automatically after a further predetermined timeperiod has passed. Thus, the user is not interrupted in his presentsurveying task, but learns that the orientation verification is to beperformed. Should the user, however, forget to manually initiate theorientation verification the orientation verification can beautomatically started. The automatically started orientationverification can begin directly after, or with a slight delay afteranother warning signal indicating the start has been issued.

Also summed up, the surveying instrument according to the inventioncomprises imaging means (also called image capturing means, e.g. acamera), a rotating means for moving the image capturing means, ahorizontal angle measuring means, a vertical angle measuring means, acontroller, and an operating means for manually operating the surveyinginstrument. The controller is adapted to calculate a horizontaldisplacement angle and/or a vertical displacement angle from adisplacement between two images of a reference object, which were takenat different times and at least one of which is stored in a memory, and,thus, to verify whether the surveying instrument's orientation at thesecond time corresponds to the orientation at the first time.

As known from the state of the art, the surveying instrument may also becontrolled in a remote manner, e.g. by a handheld data logger with radiolink to the surveying instrument body (the data logger thus working asremote controller and e.g. providing same display—andcontrol—functionalities as provided by the controlling-unit directlyattached to the instrument body). However, alternatively, the surveyinginstrument may also be remotely controlled e.g. by a laptop, a PC, a PDAor a smart phone with appropriate data linking means (e.g. also over theinternet).

Furthermore, the invention also relates to computer program product withprogram code being stored on a machine readable medium, the program codebeing configured to automatically execute and operate the inventivemethod for verifying a surveying instrument's external orientation asexplained above, particularly if the program is carried out on a controlunit of the surveying instrument.

Exemplary embodiments of the invention will be described on the basis ofthe attached figures.

FIGS. 1 and 2 are diagrams for generally explaining the determination ofa surveying instrument's orientation,

FIG. 3 a is an illustration of a first image captured while performingan embodiment of the invention,

FIG. 3 b is an illustration of a corresponding second image capturedwhile performing the embodiment of the invention,

FIG. 4 a is an illustration of a first image captured while performing apreferred embodiment of the invention,

FIG. 4 b is a detail of FIG. 4 a,

FIG. 5 is an illustration of a corresponding second image captured whileperforming the preferred embodiment of the invention, and

FIG. 6 shows how—in principle—a pixel offset in an image may betransformed into a direction/angle offset.

Exemplary embodiments of the invention will be described on the basis ofFIGS. 3 to 5.

FIG. 3 a is a schematic illustration of a first image captured by acamera mounted on a total station being a surveying instrument accordingto the invention, when the total station is directed in a predeterminedangle, for instance 100.000 Gon (1 Gon corresponds to 1/400 of a fullcircle. Thus, 100.000 Gon corresponds to a right angle.).

The captured image shows a summit cross 2 which is a reference objectaccording to the invention. The image is stored in a memory provided atthe total station together with a measured direction angle. For thispurpose, crosshairs 1 of a camera of the total station is aimed on thesummit cross 2, that is, the crosshairs 1 are exactly aimed at the basepoint of the summit cross's 2 vertical bar.

Furthermore, according to a particular embodiment of the inventivemethod where the global position of the surveying instrument is alsodetermined in advance, at the same time the orientation of the totalstation is determined, that is, the angle between the north direction(x-axis) of a global coordinate system and the direction zero of thesurveying instrument is determined using the known coordinates of thesummit cross 2 and the beforehand determined coordinates of the totalstation.

For determining the coordinates of the total station, variouspossibilities are known. For instance, the position can be determined byplacing the total station on a beforehand surveyed point the coordinatesof which are known, by determining the coordinates using surveyingmethods such as resection or intersection, taking the coordinates from amap or others. However, it has to be noted that an orientation valuedoes not necessarily have to be initially determined for theverification method according to the invention. Rather, the basic pointof the invention is to provide for a method, with which a change oforientation in the setup of the surveying station has occurred duringmeasurements.

After the orientation of the surveying instrument is initiallydetermined or just given according to the set-up of the instrument inthe field or on the street, the user starts performing the surveyingtasks. Then, after a predetermined time period has passed, a controllerprovided with the total station automatically causes the total stationto be directed to the previously measured horizontal angle (for instance100.000 Gon) and to take a second image schematically shown in FIG. 3 b,to verify the orientation of the total station.

Alternatively, the same process can be initiated upon a manual commandby the user. This is especially useful for avoiding an interruption ofthe present surveying task. Especially it is preferred, that thecontroller gives a signal after the predetermined time period haspassed, upon which signal the user may finish the present surveying taskand then initiate the automatic verification of the orientationmanually. Should the user forget to initiate the automatic verificationof the orientation, the controller can submit another reminder signal,or can even start to automatically verify the orientation after anotherappropriate time period has passed.

As can be seen from FIG. 3 b, the crosshairs 1 are not any longertargeted onto the summit cross 2 but is slightly displaced. Thus, theorientation of the total station has obviously changed. The change ofthe orientation, that is the horizontal displacement, or the verticaldisplacement as well as the change of the inclination can be calculatedon basis of the reference point's displacement in the image.

According to a presently preferred embodiment, in order to exactlyverify the displacement of the total station and, thus, the change ofthe orientation, a template matching method is applied. Such a templatematching method will be described on the basis of FIGS. 4 a, 4 b and 5.

In FIG. 4 a, the crosshairs 1 aim on a base point 6 of the summit cross.The base point 6 corresponds to a reference point. As can be taken fromFIG. 4 a, an image section 5 corresponding to an area surrounding thecrosshairs in FIG. 4 a is defined as template 5. This template 5 isdefined, when the orientation of the total station is determined for thefirst time. Furthermore, the template 5 is stored in a memory providedat the total station.

FIG. 4 b shows the exact details of the template 5. Reference sign 6corresponds to the reference point, which is the position of thecrosshairs 1 in the first image and is also stored in the memory.

The orientation is to be verified automatically after a predeterminedtime, or, alternatively, manually upon a command from the user after areminder signal to perform the orientation verification has been issued.Then, driven by a motor corresponding to a rotating means according tothe invention, the total station directs itself to the direction takenwhen the orientation was determined and the template 5 was defined. Thatis, the total station takes the same angle as before when theorientation was determined. In the example mentioned with reference toFIGS. 3 a and 3 b, the angle was 100.000 Gon. Then, a second image shownin FIG. 5 is taken and in FIG. 5 the image section corresponding to thetemplate 5 as well as the reference point 6 is determined. Furthermore,in the image of FIG. 5, the image coordinate data of the reference point6 are determined.

Then, on the basis of the image coordinate data of the reference point6, a horizontal displacement, a vertical displacement and, furthermore,a possibly occurred inclination of the total station compared to theinitial orientation can be determined. If the displacement exceeds apredetermined value which can be set by the user, furthermore, an alertsignal is issued and the user can take countermeasures.

As a possible countermeasure, for instance the displacement angle can beadded to the original orientation angle. Then, the corresponding updatedorientation angle can be used for the following surveying tasks. Anotherpossibility is to newly determine the orientation and to continue thesurveying work with the new orientation, or to return the surveyinginstrument back to the initial orientation.

While in the embodiment template matching was described as method fordetermining the displacement, it is to be noted that any other suitableimage analyzing methods can be applied. For instance, it is possible toextract a vertical edge of the summit cross 2 of FIGS. 3 a and 3 b andto determine the displacement on the basis of this edge extraction. Thisis especially applicable in a case, where it is sufficient to merelydetermine the horizontal orientation displacement.

Also, while the embodiment was described for a case where the coordinatedata of the total station are known, it is to be noted that theknowledge of the coordinate data of the total station's position are notnecessary to carry out the invention. That is, when working in a localcoordinate system chosen by the user, any arbitrarily chosen object mayserve as reference object, and its coordinate data are not necessarilyto be known for performing the invention. Again, the basic point of theinvention is to provide for a method, with which a change of orientationin the setup of the surveying station has occurred during measurements.Hence, a value (zero direction) for the external orientation does notnecessarily have to be initially determined for the verification methodaccording to the invention.

FIG. 6 shows how—in principle—a pixel offset (T′ from H) in an image(taken with camera sensor IP) may be transformed into a direction/angleoffset a (without knowledge of a distance to the target or the exactabsolute position of the surveying instrument). Therein, the lenses ofthe surveying instrument's aiming device (e.g. telescope) may be atposition P and may define an aiming axis PA (the aiming axis PA—in theshown case—coinciding with the imaging direction of the on-axis camerasensor IP). The aiming axis may be projected onto the sensor IP atposition H. If the camera is calibrated (i.e. if its camera constant isknown), the offset of the position T′ (where a target point is projectedon the sensor IP) from the position H in the image can directly betransformed (or converted)—by using triangulation—into a directionoffset between the aiming axis PA and the direction given from the lensP to the target point P, the direction offset then corresponding to thedisplacement angle α.

While the invention was described on the basis of presently preferredembodiments thereof, it is to be understood that it can be performed inmany different ways. Thus, it will be readily understood, that thedescribed embodiments are not to be considered as a limitation for thescope of the invention, which is defined by the scope of the attachedclaims. E.g., the verification of the horizontal orientation can beperformed on the basis of a horizontal pixel offset and a verificationof the longitudinal tilt (vertical alignment) on the basis of a verticalpixel offset. However, according to the invention it is moreoverpossible to verify the transverse tilt of the surveying instrument onthe basis of a rotation angle of the template in the second image withrespect to the first image. Hence, a change in the rolling angle(transverse tilt) of the surveying instrument's setup may also beregistered and compensated for according to the invention. Therefore,not only the imaged positions of the reference object in the first andsecond images may be compared, but also a rotation angle of the imagedreference object (or of a template matched with the imaged referenceobject) in the first respectively the second image.

1-15. (canceled)
 16. A method for verifying a surveying instrument'sexternal orientation during a measurement process, wherein the surveyinginstrument is at least provided with imaging means and angle measurementmeans for detecting an internal photographing direction of the imagingmeans relative to a base of the surveying instrument, the methodcomprising the initialization steps of: directing the imaging means ontoa reference object and detecting a first photographing direction of theimaging means; taking a first image of the reference object in the firstphotographing direction; memorizing the first image and the firstphotographing direction as being indicative of the surveyinginstrument's external orientation, and in a proceeded state of themeasurement process the further, verifying steps of: re-directing theimaging means onto the reference object and detecting a secondphotographing direction of the imaging means; taking a second image ofthe reference object in the second photographing direction; andcomparing a first with a second imaged position of the reference objectin the first respectively the second image by image processing as wellas the first with the second photographing direction and verifying thesurveying instrument's external orientation based on disparities betweenthe first and the second imaged position and/or between the first andthe second photographing direction.
 17. A method according to claim 16,wherein: the step of re-directing is performed in such a way, that thesecond photographing direction exactly concurs with the firstphotographing direction; and the surveying instrument's externalorientation is verified based on a disparity between the first and thesecond imaged position.
 18. A method according to claim 16, wherein: thestep of re-directing is performed in such a way, that the secondphotographing direction is different from the first photographingdirection; and the surveying instrument's external orientation isverified based on a disparity between the first and the second imagedposition and based on a disparity between the first and the secondphotographing direction.
 19. A method according to claim 16, wherein:the step of re-directing is performed in such a way, that the secondimaged position of the reference object in the second image taken in thesecond photographing direction exactly concurs with the first imagedposition of the reference object in the first image; and the surveyinginstrument's external orientation is verified based on a disparitybetween the first and the second photographing direction.
 20. A methodaccording to claim 19, wherein the step of re-directing is performediteratively by successively changing the photographing direction andsuccessively taking and processing images until the imaged position ofthe reference object exactly concurs with the first imaged position 21.A method according to claim 16, wherein: the angle measurement means aredesigned for detecting a horizontal angle as internal photographingdirection of the imaging means; the step of comparing the first with thesecond imaged position comprises determining a horizontal pixel offsetbetween the first and the second imaged position; and the surveyinginstrument's horizontal external orientation is verified based on thehorizontal pixel offset.
 22. A method according to claim 16, wherein:the angle measurement means are designed for detecting a horizontalangle and a vertical angle as internal photographing direction of theimaging means; the step of comparing the first with the second imagedposition comprises determining a horizontal pixel offset and a verticalpixel offset between the first and the second imaged position; and thesurveying instrument's horizontal and vertical external orientation isverified based on the horizontal pixel offset and the vertical pixeloffset.
 23. A method according to claim 16, wherein the step ofverifying the surveying instrument's external orientation furthercomprises determining an amount of disorientation of the surveyinginstrument, the amount of disorientations particularly being horizontaland/or vertical displacement angles, based on the disparity between thefirst and the second imaged position and/or the disparity between thefirst and the second photographing direction, respectively;
 24. A methodaccording to claim 23, wherein the amount of disorientation is used forcorrecting the value of the current surveying instrument's externalorientation and/or a warning signal is issued if the amount ofdisorientation extends beyond a preset value.
 25. A method according toclaim 16, wherein the steps of taking the first and the second imagecomprise capturing a first series of images of the reference object inthe first photographing direction respectively capturing a second seriesof images of the reference object in the second photographing directionand averaging information derivable from the images of the first seriesrespectively the second series to take place of information derivablefrom the first respectively the second image.
 26. A method according toclaim 25, wherein the positions of the reference object in eachindividual image of the first series are averaged and the positions ofthe reference object in each individual image of the second series ofimages are averaged and the averaged positions are taken as the firstrespectively the second imaged position.
 27. A method according to claim16, wherein the step of comparing the first with the second imagedposition of the reference object in the first respectively the secondimage is carried out by template matching;
 28. A method according toclaim 27, wherein: a predetermined area around the reference object inthe first image is defined as a template; the template is matched to thereference object within the second image; and a displacement of thetemplate between its position in the first and the second image isdetermined.
 29. A method according to claim 28, wherein an amount ofdisorientation of the surveying instrument is derived from thedisplacement of the template.
 30. A method according to claim 28,wherein the surveying instrument comprises a telescope defining anaiming-axis and having integrated the imaging means, wherein crosshairsare superimposable indicating the aiming-axis in the image; and thepredetermined area is defined around the crosshairs in the first imageas the template and the centre of the crosshairs representing theaiming-axis in the first image is set as a reference point of thetemplate; the template is matched within the second image; and a pixeloffset between: the position of the centre of the crosshairs in thesecond image; and the position of the reference point of the matchedtemplate in the second image, is determined indicative of adisorientation of the surveying instrument.
 31. A method according toclaim 16, wherein the step of comparing the first with the second imagedposition of the reference object in the first respectively the secondimage is carried out by an edge extraction method.
 32. A methodaccording to claim 16, wherein a reminder signal is issued after apredetermined time period has passed since the initial determination ofthe surveying instrument's external orientation or since the lastorientation verification has been carried out, and the verifying stepsare carried out upon a manual command from a user or automatically aftera further predetermined time period has passed.
 33. A method accordingto claim 16, wherein the verifying steps are repeated during themeasurement process in predetermined time intervals, particularlywherein the verifying steps are automatically carried out in a repeatedmanner in the predetermined time intervals.
 34. A method according toclaim 33, wherein: for each verification of the surveying instrument'sexternal orientation an amount of disorientation is determined and anorientation-function of time is derived from the amounts ofdisorientation, particularly by a regression analysis, especially bylinear regression, wherein the surveying instrument's externalorientation is plotted over time; and measurement results gathered inconnection with the measurement process are corrected dependent on itsrespective gathering point of times and the derivedorientation-function.
 35. A surveying instrument, comprising: imagingmeans; angle measurement means for detecting an internal photographingdirection of the imaging means relative to a base of the surveyinginstrument; operating means; a control unit, wherein the surveyinginstrument is provided with a semiautomatic or full-automaticfunctionality for verifying the surveying instrument's externalorientation during a measurement process, the control unit thereforebeing adapted to execute the initialization steps of: directing theimaging means onto a reference object and detecting a firstphotographing direction of the imaging means; taking a first image ofthe reference object in the first photographing direction; memorizingthe first image and the first photographing direction as beingindicative of the surveying instrument's external orientation, as wellas to execute, in a proceeded state of the measurement process, thefurther, verifying steps of: re-directing the imaging means onto thereference object and detecting a second photographing direction of theimaging means; taking a second image of the reference object in thesecond photographing direction; and comparing a first with a secondimaged position of the reference object in the first respectively thesecond image by image processing as well as the first with the secondphotographing direction and verifying the surveying instrument'sexternal orientation based on disparities between the first and thesecond imaged position and/or between the first and the secondphotographing direction.
 36. A tangible computer program storage productwith program code being stored on a machine readable medium, the programcode being configured to automatically execute and operate the methodfor verifying a surveying instrument's external orientation according toclaim
 16. 37. A theodolite or total station comprising the tangiblecomputer program storage product of claim 36.